1. Field of Invention
This invention relates generally to chairs adapted to accommodate an individual in a manner avoiding adverse physical side effects; and more particularly to a chair ergonomically designed to cause its occupant to assume a sitting posture which obviates undue stress on the lower back and hence avoids lower back pain.
2. Status of Prior Art
Human factors engineering, also known as ergonomics, deals with interaction which take place between an individual and a device or machine he uses or operates. The objective of ergonomic design is to attain an optimum relationship between this individual and the machine or device. Thus in designing a kitchen utensil having a handle, an ergonomic design is one making it possible for an individual who is handicapped to be able to firmly grasp the handle despite the weakness of his grip.
In the ergonomic design of a chair, the objective is not only to provide its occupant with a comfortable seat, for its main goal is to avoid impairment of the occupant's well being. Of primary concern in this regard is the avoidance of lower back pain, a condition which though widespread in a chair-sitting society, is difficult to treat effectively.
Whether back pain is treated by such disciplines as orthopedics, neurology or neurosurgery, or by osteopathic or chiropractic practitioners, positive results are not often attained. An important cause of lower back pain are the chairs which induce this condition: an individual, who after being treated medically for lower back pain, returns to the chair responsible for this condition, experiences a recurrence of the back pain.
According to U.S. Pat. No. 4,331,360, faulty posture which results in an abnormal forward curvature of the spine in the lumbar region (lordosis) is the most common cause of lower back pain. Many chairs, identified as posture chairs, because they seek to avoid faulty posture, are expressly designed so that the lumbar curve of the seated individual is supported by the chair. Thus U.S. Pat. No. 5,735,574 discloses a posture chair having a lumbar support assembly movable in the fore and aft directions at the rear of the seat and being pivoted at a back support. U.S. Pat. No. 4,331,360 maintains that the theory that the lumbar curve must be supported by the back of a chair in order to avoid back pain.
To understand the etiology of lower back pain and why existing chairs are ineffective in avoiding conditions giving rise to such pains, one must first consider the structure of the spinal column and the form it assumes when an individual is sitting in a chair.
The spinal column, as shown in FIG. 1, is typically divided into four regions: cervical; thoracic; lumbar; and sacral. When viewed laterally, each of the four regions is described as forming a curve: the cervical and lumbar being convex towards the anterior; the thoracic and sacral being concave towards the anterior. These spinal curves permit humans to walk and stand as they do. The spine itself consists of vertebrae which comprise a bony body and bony processes, two transverse processes that project laterally and one spinous process that projects posteriorly. The purpose of the transverse processes is for attachment of muscles that support the spine. These processes and the body form the borders (or rim) of a canal which contains and protects the spinal cord. As shown in FIG. 1, the vertebral bodies that make up the lumbar region have a convex curve towards the anterior, as shown by line A. However, the spinous processes in the lumbar region terminate along line B, and hence in the posterior portion of the lumbar region there is no curve at the margin of the spine defined by the spinous processes but rather the margin defines a straight line.
The spine terminates in a non-movable joint at the pelvis, a bony ring made up of the sacrum of the spine, an ilium on each side, an ischium on each side and a pubis on each side. All of these bones are fused into a rigid ring; there is no movement between the pelvis and the sacrum (i.e., the sacroiliac joint). On each side of the pelvis is a socket (acetabulum) in which the head of the femur is inserted. When a human is standing, the weight of the upper body is vectored from the spine through the ilia to the hip joints; the ischia do not bear any of the weight of the body. When viewed from the side, a straight line can be drawn from the acetabulum through the superior portion of the iliac crest to the first cervical vertebra
When an individual is seated in a chair so that his buttocks rest on the seat, then the load of his upper body weight is imposed on the ischia, or to be more exact, on the ischeal tuberosities (tuberosities are protuberances on the ischium).
The iliopsoas musculature extending between the femurs, pelvis, and the lumbar vertebrae acts to maintain the anterior lumbar curve when an individual is standing (i.e., an erect posture). In a standing individual, these lower back muscles exert forward tension on the lumbar spine to maintain its normal lordotic curve.
But while nature's design of the spinal column and its associated musculature is compatible with the postures normally assumed by primitive man who when not standing either rests on his haunches (squats) or sits cross-legged on the ground, nature did not anticipate the invention of the chair or bench, which compels its occupant to assume a sitting posture in which his buttocks rest on a seat elevated above the ground, with his thighs then extending forwardly from the seat, and his legs dangling from his thighs. The resultant sitting posture of the occupant of the chair is such that the load imposed by his upper body weight is mainly applied to the ischial tuberosities, thereby relaxing the associated iliopsoas musculature, whose function is to maintain the lumbar curve, causing relatively heavy stresses on the lumbar facet joints. It is these stresses which in time lead to lower back pain.
When stresses on the lower back are sustained for a prolonged period, as with an individual seated before a TV set for several hours, or before a computer terminal for most of a working day, lower back pain is then likely to develop to a degree that it then becomes difficult to alleviate. Moreover even if the lower back pain is successfully treated medically, it will recur should the individual return to the chair which caused the pain, or should the individual then occupy a chair similarly defective, ergonomically.
In the ergonomic design of a chair one must also take into account the gender factor. The reason for this is that in a male, the upper body weight relative to his weight below the waist is greater than in a female whose weight below the waist is more substantial that is, males have heavier upper body musculature and bones than females. Hence a chair, appropriate ergonomically for a male occupant, may not be as suitable for a female occupant.